1. Field of the Invention
This invention relates to the field of breaking emulsions and more particularly to method and apparatus for treating oil-in-water emulsions to separate oil and water therefrom.
2. Description of the Prior Art
The removal of oil from industrial waste waters is a problem of major current interest, particularly in the petroleum industry. Oil field produced waters, tanker ballast liquid, and refinery effluents are examples of fluids which may contain oil-water mixtures. The oil contained in such mixtures may be of significant value if separated and recovered. Additionally, and in some cases more importantly, if the oil is removed from the mixture, the water phase may be suitable for return to natural streams, lakes, or oceans. This can save large expense required to dispose of oil-containing water by alternate means.
No uniform governmental standard presently exists for the maximum oil content of liquid waste which may be discharged into natural waters. However, in some areas proposed pollution control regulations set a maximum limit of 30 ppm oil.
In some cases, oil and water may be adequately separated to meet the proposed standard merely by allowing the mixture to sit for a short time in a quiescent settling zone where water sinks to the bottom and oil rises to the top. In other cases, the oil and water form an emulsion in which one of the liquids is relatively stably distributed in the form of disperse droplets in the other liquid, the "continuous phase". Usually such emulsions will not separate by mere settling. Further treatment is required to "break" them into their constituent liquids.
Exemplary prior art methods for separating emulsions into separate components include imposing electrical fields on the emulsion to cause dispersed drops to rapidly collide with one another and thereby coalesce into larger drops which may be more easily separated from the mixture. Similar results have been achieved by adding chemical agents or "demulsifiers" which promote coalescense of dispersed droplets into larger drops. These drops can then be separated from the continuous phase by settling.
Another method of separating disperse phase from an emulsion is to contact the emulsion with a solid which is preferentially disperse-phase wet. For example, in an oil-in-water emulsion, oil is collected on an oil-wet material and coalesced into masses of such size as will readily stratify, e.g., see U.S. Pat. No. 1,887,774 to Meinzer, issued Nov. 15, 1932, U.S. Pat. No. 3,405,059 to Sprow, issued Oct. 8, 1968, and U.S. Pat. No. 3,152,196 to Marziani, issued Oct. 6, 1964. In these prior techniques it usually has been necessary to pass a mixture of water and coalesced oil droplets to a separate settling zone or through separating means in which liquids segregate due to density differences.
It is also known to break emulsions by agitating the emulsion with a disperse-phase collecting solid. For example, with an oil-in-water emulsion, it is known to separate the oil from the emulsion by agitating with an oil attracting solid. The solid and attached oil is then removed from the mixture by filtration, settling or other means. The solid can be discarded as waste or separately treated to remove oil and then reused. Patents exemplary to this technique are U.S. Pat. No. 705,253 to Krause, issued July 22, 1902, U.S. Pat. No. 3,147,216 to Oemler, issued Sept. 1, 1964, and U.S. Pat. No. 3,580,844 to Fratzsher et al, issued May 25, 1971.
None of these prior art methods has been entirely effective to date in all applications. Collection of oil on oil-attracting solids has the disadvantage of creating oil-soaked solid waste material which must in turn be treated or disposed of. Processes which require quiescent settling zones are disadvantageous where large volumes of fluid must be treated. In many such applications, such processes may be entirely adequate with respect to efficiency in separating oil from water given sufficient time. However where great amounts of fluid are involved, as in industrial operations of the type mentioned above, provision of containers to hold liquids to be treated during settling may require prohibitively large capital investment.
For example, an offshore oil production facility may handle as much as one hundred thousand or more barrels per day of produced water. Using conventional settling techniques these waters can often be reduced in oil content to about 1000 ppm in a relatively short time. However using prior emulsion breaking techniques to separate the remaining oil content, which often is in the form of dispersed drops on the order of 1-10 microns in diameter, may take additional settling time of several days up to weeks or more. Facilities for handling such large volumes of fluid are rarely available on offshore platforms.
If the oil contained in this water could be continuously separated, the water might be suitable for return to the ocean or other body of water in which the platform is located. However, there is no currently available continuously operating commercial emulsion breaking process which can attain the 30 ppm oil concentration required by proposed governmental regulations. Therefore, expensive alternative provisions must be made for disposing of this oil-containing water.